zone mydomain.example/IN: 'mydomain.example' found SPF/TXT record but no SPF/SPF record found, add matching type SPF record
zone other.domain/IN: 'other.domain' found SPF/TXT record but no SPF/SPF record found, add matching type SPF record

The mechanism are evaluated in order. You can use +mx but this makes no sence if you use ~all for the rest.
I suggest to keep spf-records simple. Just use mx (and maybe a), never use ptr (see rfc7208), prevent includes and set your mechanism at the end.
I would not use "v=spf1 +a +mx -all". Keep it simple and use "v=spf1 a mx -all" (+ is the default).

The spf-record depends on your server / ips.
If you have only one IP and have a MX-Record for this IP: v=spf1 mx ~all
If you use additional MX-Server to send Mail, add them: v=spf1 mx mx:2nd.example,com ~all
If you send mails from IPs that have no MX record but an A-record: v=spf1 mx a ~all or v=spf1 mx a:1.2.3.4 ~all